Why this matters for paywalled content
Google allows "flexible sampling" paywalls where search crawlers and visitors both see a preview, but it explicitly prohibits showing crawlers the complete article while showing regular visitors only a locked preview - that is treated as cloaking, which can lead to a manual action against the site. This fetches your page once with a Googlebot user agent and once with a regular browser user agent, then compares how much visible text each version actually contains.
Why a small difference is normal, but a large one is the pattern worth investigating
A small difference is normal and expected for a legitimate metered or preview paywall. A large difference - where Googlebot sees dramatically more content than a real visitor - is the pattern that matters: it's the specific signature of a paywall implemented by detecting the Googlebot user agent and serving it the full article, rather than by the documented flexible-sampling approach Google permits.
Why the tool reports a character count for each version rather than just a pass/fail verdict
Showing the actual character counts (and a short text preview) for both the Googlebot and the regular-visitor version lets you see the specific gap for yourself, rather than trusting a single verdict label blindly - useful for judging whether a flagged difference is genuinely the full article being hidden, or just a longer preview than expected.
Why this checks the page's HTML directly rather than relying on the page's own schema markup
A page could claim to use flexible sampling and isAccessibleForFree structured data correctly
while still serving crawlers meaningfully more content than visitors in practice - comparing what each user
agent actually receives, rather than trusting what the markup claims, is what catches a real implementation gap
between the two.